Hair cracks on the surface are due to shrinkage, and are worse the richer the face mortar, and are worse with wet than with dry concrete. These cracks are only the width and depth of a coarse hair, and do not materially weaken the concrete; but they do seri ously disfigure a smooth concrete surface. Often they do not appear for several weeks after the concrete has set. Excessive troweling brings to the surface water which carries with it the most finely ground portions of the cement, and makes the surface mortar richer, and consequently increases the liability of surface cracks. These hair cracks or "map lines" or "crazing of the surface" may usually be prevented by keeping the surface wet for a considerable time.
Spade Finish. One of the advantages of a wet over a dry concrete is that the former will flow against the form and give a more solid surface. With plastic or wet concrete a solid surface is insured by forcing a flat-blade spade vertically down between the concrete and the form, and then pulling the top of the spade away from the form. This forces the coarse fragments back from the face and allows the mortar to flow against the form. A perforated or a fork like spade is sometimes used in this work.
Sometimes it is desired simply to whitewash a surface to secure a uniform color, in which case the following formula may be useful. It has long been used for both in side and outside work, and gives a coating that resists wear well and that retains its brilliancy for years.
Slack with warm water half a bushel of lime, covering it during the process to keep in the steam; and strain the liquid through a fine sieve or strainer. To the slaked lime add the following: 1 peck of salt previously well dissolved in warm water, 3 pounds of ground rice boiled to a thin paste and stirred in boiling hot water, pound of powdered Spanish whiting, 1 pound of glue which has been pre viously dissolved over a slow fire, and 5 gallons of hot water. Stir well and let 'the mixture stand for a few days, covered from dirt. Strain carefully and apply hot with a brush or a spray pump. Color ing matter may be put in to make almost any shade.
Discoloration and small honeycomb spots and the marks of the grain of the wood forms can be obliterated by applying to the surface a wash of neat portland-cement grout. The grout should be mixed to about the consistency of thick cream, and should be applied with a whitewash brush or an old broom, worked perpendicularly to the horizontal joints of the forms. The color of the finished surface will be considerably lighter if plaster of paris be substituted for about one quarter of the cement.
Of course any considerable holes in the surface should be plastered up with a 1 : 2 or 1 : 3 mortar before applying the wash.
When, after removing the forms, the surface of the concrete is spotted, honeycombed, and has holes in it because of the adhesion of the concrete to the forms, the attempt is some times made to plaster the surface with a coat of cement mortar; but it is nearly impossible to make the plaster coat adhere firmly to the set concrete. Owing to the difficulty of getting the coat to adhere, it is unwise to attempt to plaster a surface simply to improve its appearance, although a plaster coat is sometimes applied to make a wall waterproof (see I 363).
To insure a good plaster face upon the concrete stadium at Syracuse, N. Y., wire nails were driven at frequent intervals into the forms so as to project from the concrete when the forms were removed; and then after the concrete had set and the forms had been removed a washer was placed upon each projecting nail and sheets of wire lathing were placed against the face of the concrete and fastened in position by bending down the projecting end of the nail, the washer keeping the lathing a little distance from the concrete. The plaster ing was then applied to the, wire lathing.
The following method is effective in removing the marks of the forms and is not expensive, provided it is applied while the concrete is still green, say, 24 to 48 hours old. As soon as the forms are taken down, the concrete is rubbed with a soft brick or a block of wood, taking care to use plenty of water either by dipping the brick or block into a pail of water or by throwing the water on the wall with a whitewash brush or small broom.
The purpose of the preceding methods is to secure a uniform and smooth surface; but an artistic effect can be produced by proceeding in the opposite direction. A facing 2 or 3 inches thick of dry concrete composed of 1 part cement, 3 parts of sand or screening, and 3 parts of }- or finch pebbles or broken stone may be applied by either process described in 4 351. The facing should not be spaded, as it should be mixed too dry to permit any flushing of the mortar. The surface should be evenly grained and finely honeycombed, the imprint of the joints between the planks of the form should scarcely be noticed, and the grain of the wood should not show at all. There is no efflorescence on such a surface. This surface is much used in the buildings of the South Side Parks in Chicago with entire satisfaction.